1. Field of the Invention
The present invention relates to a package for an optical fiber Bragg grating that facilitates accurate fiber adjustment and temperature compensation at selected frequencies. More particularly, the invention provides a packaged Bragg grating of compact size, using passive compensation for consistent wavelength response over a range of temperatures.
2. Discussion of the Related Art
Fiber Bragg gratings may be fabricated with precisely controlled transmission and reflection characteristics that are optically stable over long periods of time. A fiber Bragg grating normally comprises a repeating pattern written into a photosensitive optical fiber using a UV light source or the like. Signals associated with the modulating repeating pattern will vary in response to changes of strain and temperature that affect the physical condition of the inherently highly sensitive grating structure.
Thermal variability of fiber Bragg gratings has been one factor limiting their use in commercial applications. Telecommunications systems, for example, typically operate between temperatures extremes of about xe2x88x925xc2x0 C. to about 70xc2x0 C. The use of passive temperature compensation provides fiber Bragg gratings having consistent response over such a temperature range. Reduction of thermal variability led to more reliable devices for commercial use in e.g. high speed optically amplified transmission networks for the telecommunications industry.
One method for passive temperature compensation of fiber Bragg gratings requires the production of a grating package with a negative coefficient of thermal expansion (CTE). This is usually accomplished by clamping, under tension, the fiber containing the fiber Bragg grating into a mechanical structure made of materials having different, but usually positive, coefficients of thermal expansion. A low-expansion material, such as a ceramic, combined with a high expansion material, such as a metal, yields a package having a coefficient of thermal expansion determined by material selection and device dimensions. This method of passive temperature compensation is well known as a means for improving wavelength stability of fiber Bragg gratings. At least two variations of the method have been investigated. In one approach, materials differing in thermal expansion provide a package that varies the length of an optical fiber. The structure is arranged such that different rates of expansion between the fiber-supporting, structural members cause negative elongation of the fiber with increasing temperature. Typically the fiber is stretched at low temperature and relaxes as the temperature increases, thereby changing the stress applied to the fiber under tension. U.S. Pat. No. U.S. 5,042,898 discloses an apparatus for temperature compensation of a fiber Bragg grating comprising two juxtaposed compensating members with the required differences in thermal expansion. Attachment of the fiber to points on each of the members, places the grating between the two attachment points. The apparatus can be designed to apply tensile or compressive stress to the grating. Other references addressing temperature compensation of fiber Bragg gratings using fiber length variation include U.S. Pat. No. 5,991,483; U.S. Pat. No. 6,101,301 and International Published Application WO 98/59267. Japanese publication JP 9211348 describes the use of a piezoelectric transducer to modulate the strain in a fiber in response to electrical signals. Such devices are effective but costly.
A second variation of passive temperature compensation using materials of dissimilar thermal expansion causes changes in the bend radius of packaged fiber Bragg gratings. This produces tensile stresses in the region of the grating to counterbalance and compensate for wavelength variations resulting from changes in the grating temperature, as described in U.S. Pat. No. 5,841,920 and U.S. Pat. No. 6,044,189.
Temperature compensated fiber Bragg grating packages, as previously discussed, are typically large, exhibiting variation of reflection wavelength from one package to another. In some cases, the design of temperature compensating structures is complex requiring multiple points of connection to form a package having a negative coefficient of thermal expansion. Some packages include fine adjustment of the grating wavelength but this may involve complicated procedures such as the extension or compression of the total package as described in WO 98/59267.
Accordingly, there is need for a small, simple and inexpensive device to provide passive temperature compensation and precise control of fiber Bragg grating characteristics using active strain adjustment to set the desired initial wavelength of a grating during manufacture.
The present invention provides an improved, compact temperature compensated fiber Bragg grating package and a method for its manufacture including fine tuning the center wavelength of a fiber grating either during or after manufacture of the package.
A temperature compensated fiber Bragg grating package, according to the present invention, includes a fiber support comprising a first member having a first coefficient of thermal expansion and two second members, one each attached adjacent to the ends of the first member. The second members have a second coefficient of thermal expansion that is relatively more than the first coefficient of thermal expansion. The grating package further includes an optical fiber attached to a fiber support between the two second members. A fiber Bragg grating, formed in the optical fiber, may be tuned to a selected wavelength and be provided with temperature compensation means for compensating for any fluctuations in temperature of the package. This is accomplished by adjusting both the length of optical fiber between the second members and the relative positioning of the second members adjacent to the ends of the first member. A compact fiber Bragg grating package, according to the present invention, has a length less than 16 cm. In more compact devices packages having a length less than 10 cm may be selected.
Fiber Bragg gratings, according to the present invention meet requirements for use in an operating temperature range of from about 0xc2x0 C. to about 60xc2x0 C., preferably form about xe2x88x925xc2x0 C. to about 70xc2x0 C. This range could be further refined to about xe2x88x9220xc2x0 C. to about 80xc2x0 C. Design requirements also typically require storage temperatures in the range from about xe2x88x9245xc2x0 C. to about 85xc2x0 C.
More particularly the present invention provides a fiber grating package comprising a rod having a first end and a second end and a coefficient of thermal expansion. A first end cap includes a first base having a first opening formed therein to receive the rod for movement of the first end cap along the rod to a first position adjacent to the first end of the rod. The first end cap further includes a first cantilever member extending from the first base and the first cantilever member has a first contact point thereon. A second end cap includes a second base having a second opening formed therein to receive said rod for movement of said second end cap along said rod to a second position adjacent to the second end of the rod. The second end cap further includes a second cantilever member extending from the second base and the second cantilever member has a second contact point thereon. The first end cap and the second end cap have a common rate of thermal expansion that is greater than the coefficient of thermal expansion of the rod A portion of an optical fiber includes a Bragg grating, the portion of the optical fiber being attached between the first contact point and the second contact point. The portion of an optical fiber has a length defined by the distance between the first and second contact points, such that the length of the portion remains substantially unchanged in an operating range of temperature when the first end cap occupies the first position and the second end cap occupies the second position.
The present invention includes a method for assembling and then tuning a temperature compensated fiber grating package. The method includes providing a rod having a first end and a second end and a coefficient of thermal expansion and mounting a first end cap including a first base at a first position adjacent to the first end of the rod. The first end cap further includes a first cantilever member extending from the first base and the first cantilever member has a first contact point thereon. A second end cap including a base is then mounted at a second position adjacent to the second end of the rod. The second end cap further includes a second cantilever member extending from the second base and the second cantilever member has a second contact point thereon. The first end cap and the second end cap have a common rate of thermal expansion that is greater than the coefficient of thermal expansion of the rod. A portion of an optical fiber including a Bragg grating is attached between the first contact point and the second contact point. The portion has a length defined substantially by the distance between the first and second contact points. Thereafter a torsional force is applied to at least one of the cantilever members to increase the distance between the at least one cantilever member and the rod to tune the Bragg grating to a selected wavelength.
The present invention further provides a method for assembling a wavelength tuned, temperature compensated fiber grating package, comprising providing a rod having a first end and a second end and a coefficient of thermal expansion and mounting a first end cap including a first base at a first position adjacent to the first end of the rod. The first end cap further includes a first cantilever member extending from the base and the first cantilever member has a first contact point thereon. The first end is secured at the first position before mounting a split end cap including a base member adjacent to the second end of the rod. The split end cap further includes a cantilever member extending from the base member of the split end cap and detachable therefrom. The cantilever member of the split end cap has a second contact point thereon. The first end cap and the split end cap have a common rate of thermal expansion that is greater than the coefficient of thermal expansion of the rod. A portion of an optical fiber including a Bragg grating is attached between the first contact point and the second contact point. The portion of an optical fiber is tuned to a selected wavelength corresponding to a length defined by the distance between the first and second contact points. While maintaining the defined length, the base member is detached from the cantilever member and moved to a position separated from the first position such that the length of the portion of an optical fiber remains substantially unchanged in an operating range of temperature. Thereafter the base member is securely bonded to the rod and the cantilever member is securely bonded to the base member of the split end cap to produce the wavelength tuned, temperature compensated fiber grating package.